A shock absorber mounted on a vehicle may have a damping force adjusting mechanism. The mechanism does not allow sufficient riding comfort and steering safety which have a mutually opposing relationship because of being a shock absorber of which a damping force is uniquely determined with respect to an operating speed of a piston. Therefore, various types of damping force adjusting mechanism, in which the generated damping force is adjustable by the piston, are known. For example, Japanese Patent No. 3103062 (Reference 1) discloses “a vibration damper which includes a cylinder that has a damping fluid; a piston rod that is inserted into the cylinder and is sealed in and is movably disposed in an axial direction; a damping piston that is fixed to the piston rod and divides the cylinder into two operating chambers; and a damping valve that adjusts a cross-section of a damping duct of a main stage that functions due to having a valve body movable in the axial direction and includes a valve seat, and is adjustable for a motor vehicle. A valve element that is movable in the axial direction is disposed for positioning the valve body in one direction and the valve element urges the valve body to a rear side with a pressure of the damping fluid flowing therein from an operating chamber through a flow communicating portion having a throttle in a vibration damper that is adjustable for the motor vehicle. Therefore, the valve element creates a pilot control operation with respect to the valve body in the direction, the valve body is directly urged in the other direction, and the valve body is urged by a pressure of the damping fluid of the corresponding operating chamber.” (described in claim 1 of Reference 1 and omitting reference numerals quoted in figures).
In addition, Japanese Patent No. 4985984 (Reference 2) has an object to “provide a damping force adjusting type shock absorber which is able to generate a damping force that is stable even during actuator failure” and proposes a damping force adjusting type shock absorber “which includes a cylinder in which a fluid is sealed; a piston that is slidably provided within the cylinder; a piston rod that is connected to the piston and extends from the cylinder to the outside; a passage that causes the fluid to flow by the sliding of the piston within the cylinder; a pilot type damping valve that generates a damping force by controlling the flow of the fluid in the passage and adjusts a valve opening pressure using some flow of the fluid as a pilot pressure; a damping force adjusting valve that adjusts the damping force by controlling some flow of the fluid and adjusting the pilot pressure; and an actuator that operates the damping force adjusting valve, in which the damping force adjusting valve limits the flow of the fluid during actuator failure, a relief valve is provided in parallel with the damping force adjusting valve, and a sub-damping valve for controlling the flow of the fluid is provided on a downstream side of the relief valve” (described in paragraphs [0010] and [0011] of Reference 2).
In the vibration damper described in Reference 1 described above, a mechanism, in which the damping force is variable by driving the valve body by the actuator, is disclosed. However, a control valve is disposed on the damping valve and the pilot pressure control is possible with respect to the flow of the fluid in one direction, but the flow of the fluid in a reverse direction is controlled by direct driving of the actuator. Therefore, the damping force is forced to be either a maximum or minimum during failure. In contrast, if a structure including two damping valves is provided, the pilot pressure control is possible with respect to a bi-directional flow of the fluid and, as described in Reference 2, damping force characteristics during failure can be arbitrarily set, but the number of components increases, a structure becomes large and complicated, and an increase in costs is caused.